RE: Advice for a mid-range noob looking to buy a system

What is the goal?
Simple enjoyment in the pursuit?
Becoming adept in hardware?
Becoming adept in development?
Putting to use some rusty skills you once acquired in electrical engineering and computer circuitry?
I'm not knocking any of those. I'm only trying to understand where you are coming from.

If you're trying to become adept in the hardware for financial gain IDK if any of that old hardware will help you. OTOH it might as it will familiarize yourself with having to find where IBM documentation is on what hardware runs on what models, etc. But basically those running that old of hardware are playing their money very close to their chest and are probably loath to part with even pennies on those machines. So it may or may not help. I would take that advice and try to hook up with Lan console or HMC as there's no money to be made learning how to hook up twinax.

If the goal is to become adept in development I would take the advice of foregoing any hardware and using a timeshare service instead. Like the PUB400 mentioned. There are others also like https://iinthecloud.com/idev-cloud/
Using one of these allows you access to the latest versions of the OS instead of limiting your development to versions well out of date. This is the sole reason why I never kept one of our old machines for personal use. I am a junkie for the latest. There are those who thrive on developing for the oldest possible machines they can find. God bless them but that ain't my bag.

If your goal is to polish up your electrical engineering skills then have at it.

Rob Berendt